(This is a manual merge of #9102)

This is a follow up to #16024.

`DateTime#to_f` now preserves fractional seconds.

Previously this method always returns `.0` in the fractional part. This commit
changes it to preserve the fractional seconds instead.

Fixes #15994.

automatically include ActiveModel::Validations when include ActiveModel::SecurePassword

Move pk initialization logic onto `AttributeSet`

Conflicts:
	activerecord/lib/active_record/attribute_set.rb

Quote range strings when quoting PG ranges

The test case for CVE-2014-3483 doesn't actually send the generated SQL
to the database. The generated SQL is actually invalid for real inputs.

Fix CVE-2014-3483 and protect against CVE-2014-3482.

Don't error when `dup`ing a record with no PK

flash doesn't pass objects #15522 [ci skip]

Better encapsulates its internals from `ActiveRecord::Base`.

Revert "Merge pull request #15394 from morgoth/fix-automatic-maintaining-test-schema-for-sql-format"

This reverts commit 46139d33, reversing
changes made to 8f247871.

Conflicts:
	activerecord/CHANGELOG.md

Add regression tests for keys on nested associations

This adds the regressions tests from issue #15893 to
master. It's checking that both strings and symbols are
accepted as keys for nested associations.

[ci skip] Fixed the documentation of xml_http_request? (aka xhr?)

Add configuration to enable mail previews

Silence warning emitted in tests

[ci skip] Add missing prologue section of upgrading ruby on rails guide.

The instance method `model_name` was being defined multiple times,
causing a redefinition warning.

Fix doc unwanted dl Admin:: [ci skip]

Adds `config.action_mailer.preview_enabled`

This allows mail previewing to be enabled easily in non-development
environments such as staging. The default is set to true for development
so no changes should be required to existing Rails applications.

The mail preview path can still be configured using the existing
`config.action_mailer.preview_path` configuration option.

Adding this avoids devs from having to do stuff like:
https://gist.github.com/lengarvey/fa2c9bd6cdbeba96526a

Update actionmailer/CHANGELOG with new configuration.
Update configuring guide with new configuratation.
Add `config.action_mailer.preview_path` to configuring guide.

Then we only need to extract host once.

Closes #15214

Since we are using both branches of the code is preferable to use
if/else over the early return.

Added String support for min and max properties for date field helpers

To avoid overload with database rollback